1. Field of the Invention
The present invention relates to an electric vehicle power supply unit wherein DC power obtained from a battery power source is converted into AC power to power an electric motor that drives the vehicle. More particularly, the invention relates to a power supply unit that limits a rush current, which is caused when the DC power supply is switched-ON and DC power is supplied via an inverter to a motor, without the necessity of using a current limiting resistor. Further, the invention provides a simplified circuit configuration which permits an electric charge stored in the inverter's current smoothing capacitor to be discharged therefrom without the provision of a discharging circuit.
2. Description of Background Art
A power switching circuit contrived to prevent a rush current by the use of an electronic switch is proposed in Japanese Patent Laid-open No. Sho 61 (1986) 280711.
FIG. 7 is a circuit diagram showing the configuration of a conventional rush-current-preventing power switching circuit. According to the conventional circuit, when a switch 202 is closed, a capacitor 204 is gradually charged via a resistor 203 which causes NPN-type transistor 205 to be gradually placed in a conducting state. Subsequently, a PNP-type transistor 206, whose base current is controlled by the NPN-type transistor 205, is gradually placed in a conducting state. Consequently, a DC power source 207 and a load 208 (having a capacitive load) can be connected to each other without causing a rush-current.
FIG. 6 is a circuit block diagram of a known electric-vehicle power supply unit constituted by utilizing the conventional rush-current-preventing power switching circuit 201 shown in FIG. 7 and described above.
The electric-vehicle power supply unit 101 employs a PNP-type transistor 206 that is gradually placed in a conducting state in response to turn-ON of a main switch 102 as described above, whereby rush current flowing in inverter 103 and smoothing capacitor 104 is suppressed.
The electric charge stored in the current smoothing capacitor 104, however, cannot be discharged therefrom when the main switch 102 is in its OFF-state. Therefore, the prior art must provide a relay contact 105 which is placed in a conducting state in response to turn-OFF of the main switch 102 to discharge the energy stored in current smoothing capacitor 104 through the contact 105 and discharging resistor 106.
It is further necessary in the prior art to provide a diode 109 in parallel with the PNP-type transistor 206 in order to supply the regenerative power (which is generated at the time of regenerative braking of motor 107) to a battery power source 108 for absorption of such regenerative power.
Also in the conventional rush-current preventive power switching circuit 201, the PNP-type transistor 206 is kept completely in its OFF-state until the NPN-type transistor 205 is placed into operation by the charging of capacitor 204 such that the voltage thereof becomes higher than the threshold voltage between the base and the emitter of the NPN-type transistor 205. Accordingly, the actual supply of power from the DC power source to the load circuit 208 after closing of the front switch 202 is delayed. Consequently, the conventional circuit suffers from a time delay after turn-on of the main switch 102 which delays acceleration of the electric vehicle.